Can anyone help me with some custom fields issues?

Hello forum! My site is http://www.e-win.gr and i run the Habitat Theme.

My issue is that i don’t exactly know how to make a custom field that in every post the value should be another! For example, my site is has to do with competitions and i want to post an announcement about two competitions but the first competition expires in 30/7/2011 and the second competition expires in 25/7/2011 .. So i want to have in every post in this text “Ημερομηνία Λήξης Διαγωνισμού:” (this means Competition’s expiration date) and next to it should be the value i put in every post. So in the first post edit i put 30/7/2011 and in second post edit i put 25/7/2011 .. What is the exact code to do it?

Please help me! For further information please ask me to explain!

Thanks!